"Kasab is being treated for fever for the past few days. Suspecting that the prisoner could be suffering from dengue, three tests were conducted on him by a team of doctors of JJ hospital but the results were negative," said a police official on condition of anonymity.

"Doctors do not want to take any chance. Kasab is under observation and responding well to the treatment," the official said.

The Pakistani terrorist is currently lodged in a bomb proof, egg-shaped cell at the jail since his arrest. His mercy plea was rejected last month by the government, which has sent its recommendation to the President.

Kasab, the lone surviving terrorist of the 26/11 Mumbai attacks was involved in a grave crime and waging war against India that led to killing 166 people, including foreigners.

The 25-year-old Pakistani and nine other fellow Lashkar-e-Toiba terrorists had landed in Mumbai on November 26, 2008, by sea from Karachi and had gone on a shooting spree at various places, carrying out the country's worst terror attack. While Kasab was captured alive, the other terrorists were killed by security forces.

After nearly a four-year-long legal battle, on August 29 the Supreme Court had confirmed the death penalty awarded to the LeT operative by the trial court and later upheld by the Bombay High Court.